EP0865699B1 - Verfahren und einrichtung zum ausgleich von gleichspannungsverschiebungen und wiederherstellen der niederfrequenten komponenten eines basisbandsignals - Google Patents

Verfahren und einrichtung zum ausgleich von gleichspannungsverschiebungen und wiederherstellen der niederfrequenten komponenten eines basisbandsignals Download PDF

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Publication number
EP0865699B1
EP0865699B1 EP97910903A EP97910903A EP0865699B1 EP 0865699 B1 EP0865699 B1 EP 0865699B1 EP 97910903 A EP97910903 A EP 97910903A EP 97910903 A EP97910903 A EP 97910903A EP 0865699 B1 EP0865699 B1 EP 0865699B1
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EP
European Patent Office
Prior art keywords
signal
baseband
baseband signal
processing circuit
signal processing
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Expired - Lifetime
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EP97910903A
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English (en)
French (fr)
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EP0865699A1 (de
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Andrew C. Barber
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Harris Corp
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Harris Corp
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/06—Dc level restoring means; Bias distortion correction ; Decision circuits providing symbol by symbol detection
    • H04L25/061—Dc level restoring means; Bias distortion correction ; Decision circuits providing symbol by symbol detection providing hard decisions only; arrangements for tracking or suppressing unwanted low frequency components, e.g. removal of dc offset
    • H04L25/063—Setting decision thresholds using feedback techniques only

Definitions

  • the present invention relates in general to communication systems, and is particularly directed to a signal processing mechanism for reconstructing low frequency information that has been removed by high pass-filtering of a baseband signal prior to data decision processing, and canceling low frequency DC impairments to the baseband signal, such as may be introduced by mechanical stress in a printed circuit board containing demodulator components, AGC amplifier effets, microphonics, and DC offset introduced by a high gain amplifier through which the baseband signal is amplified upstream of data recovery.
  • Figure 1 diagrammatically illustrates a simplified configuration of a baseband demodulator for an RF modem having a first down-converting mixer 10, to a first input 11 of which the output of a first IF local oscillator signal is applied and to a second input 12 of which a modulated RF signal of interest is applied.
  • the output 14 of the mixer 10 is an intermediate frequency (IF) signal, which is to be down-converted to baseband for application to a data recovery signal processing circuit.
  • IF intermediate frequency
  • the IF signal which may be in the microvolt range, is coupled through an IF bandpass filter 15 to a first input 21 of a second down-converting mixer 20, which has a second input 22 coupled to receive the output of a second, baseband down-converting local oscillator, the amplitude of which typically varies in the millivolt range.
  • Mixer 20 has an output 24 that produces a baseband signal containing the modulated data of interest.
  • the modulated baseband signal is coupled to an AGC-driven, high gain amplifier 30, which amplifies the (microvolt) baseband modulation up to a higher range of voltage amplitude variation (e.g., the millivolt range) for application to a downstream signal processing circuit 40.
  • the signal processing circuit 40 compares a sample of the instantaneous amplitude of the baseband signal with one or more DC quantization levels, to extract modulation/data values associated with those quantization levels.
  • the signal processing circuit 40 may comprise a simple polarity-sensitive comparator, the output of which is one of two binary values (0/1) depending upon the polarity of the AC-coupled baseband signal sample of interest.
  • DC offset can be introduced into the baseband signal path from a number of sources including, but not limited to, mechanical stress in the printed circuit board containing the components of the demodulator circuitry, microphonics, and a DC offset introduced by the high gain amplifier.
  • the output 34 of the amplifier 30 is coupled to the signal processing circuit 40 through a further high pass-filter (capacitor) 39.
  • the signal processing circuit comprises a post-processing combiner, a feedback path including an integrator, and a pre-processing combiner.
  • WALDHAUER 'Quantized feedback in a experimental 280-mb/s digital repeater for coaxial transmission' IEEE TRANSACTIONS ON COMMUNICATIONS; vol. com-22, no. 1, January 1974, pages 1-5, XP002055332 discloses a quantized feedback in a digital repeater for coaxial transmission, in which baseline-wander is cancelled by an equal and opposite signal derived from the repeater output.
  • the pulse stream is quantized to a pre-determined level by a regenerator, passed through a linear filter that is complementary to the forward-path low-frequency cut-off, and added to the incoming signal ahead of the regenerator.
  • the above-described information loss and DC offset impairment problems associated with AC-coupling the signal processing path of an RF modem are effectively obviated by a low frequency reconstruction or reinsertion and DC offset cancellation signal processing methodology.
  • the quantized data output of a baseband slicer/comparator is differentially combined in an output combiner with the baseband signal to produce a wideband error signal. This error signal is integrated to derive a DC correction component.
  • This DC correction component is fed back to a pre-processor combiner, which differentially combines the DC correction component with the AC-coupled baseband signal.
  • the output of the pre-processor combiner is thus the baseband signal of interest, absent the slowly varying DC offset error component, so that the amplitude values of the signal constellation input to the quantization circuitry of the data recovery processor are not distorted by the slowly varying DC offset level upon which the baseband modulation is riding.
  • the quantized data output of the slicer/comparator is coupled to a low pass filter whose upper cut-off frequency substantially corresponds to the lower cut-off frequency of the high pass filtering (capacitive AC-coupling) of the baseband signal upstream of the comparator.
  • the effect of the low pass filter is to recover an exact representation of the low frequency signal component of the baseband signal that had been removed by the AC-coupling of the baseband signal prior to its application to the signal processing comparator.
  • the output of the low pass filter is coupled to a summing input of the pre-processor combiner, so as to be summed with the AC-coupled baseband signal (absent its removed low frequency component). Namely, by virtue of the low pass filter feedback path, the previously removed (low frequency) portion of the useful frequency spectrum of the baseband signal is reconstructed, and injected by the pre-processor combiner into the baseband signal path prior to its application to the comparator.
  • the signal processing circuit 40 comprises a comparator which is operative to compare a sample of the instantaneous amplitude of the amplified and AC-coupled baseband signal with one or more DC quantization levels, to extract modulation/data values associated with those quantization levels.
  • the quantized data output 44 of slicer/comparator 40 is coupled to a first input 51 of a differential combiner or subtractor 50, a second input 52 of which is the baseband signal applied that is applied to the input 41 of comparator 40.
  • the output 54 of differential combiner 50 is representative of the quantized/sliced data values produced by the comparator 40 and its input baseband sample values.
  • These sampled signal amplitude differences constitute wideband error signals, which include the slowly varying DC component of the baseband signal.
  • slowly varying is meant a gradual rate of change approximate DC or on the order of only several Hz up to 10 Hz, as a non-limiting example, as opposed to a higher rate of change that may generally be classified as Gaussian noise.
  • This slowly varying wideband error signal produced by differential combiner 50 is coupled via a first fractional bandwidth feedback path 56 to an integrator 60, which has a high gain in the low frequency bandwidth of interest (e.g., 10 Hz., as a non-limiting example) and a substantially reduced gain above this range. Integrator 60 integrates the errors and thereby derives a DC correction component representative of the accumulated variation of the slowly varying wideband error signal.
  • the DC correction component is fed back to a first differential (-) input 71 of a pre-processor summing unit 70, which has a second (+) input 72 thereof AC-coupled to the output of amplifier 30, so that the signal input 72 of summing unit 70 is the high pass-filtered baseband signal of interest, riding on a slowly varying DC offset, such as may be introduced by mechanical stress in the printed circuit board, AGC amplifier effects, or microphonics effects, as described above.
  • the output 74 of summing unit 70 is thus the difference between the baseband signal and the DC correction component produced by integrator 60, namely the high pass-filtered baseband signal of interest, absent the slowly varying DC offset error component.
  • the quantized data output 44 of slicer/comparator 40 is coupled via a second fractional bandwidth feedback path 58 to a low pass filter 80 having an upper cut-off frequency (e.g., 10Hz), that substantially corresponds to the lower cut-off frequency of the high pass filtering (capacitive AC-coupling) of the baseband signal upstream of the comparator 40.
  • the effect of low pass filter 80 is to recover an exact representation of the energy content of the low frequency signal component of the baseband signal that had been removed by the AC-coupling of the baseband signal prior to its application to the signal processing circuit 40.
  • the output of low pass filter 80 is coupled to a third (+) input 73 of summing unit 70, so as to be summed with the AC-coupled baseband signal (absent its removed low frequency component). Namely, by virtue of the low pass filter feedback path, the previously removed (low frequency) portion of the useful frequency spectrum of the baseband signal is reconstructed, and injected (via summing unit 70) into the baseband signal path prior to its application to comparator 40.
  • a low frequency reconstruction and DC offset cancellation signal processing methodology which is operative to reconstruct low frequency information that has been removed by high pass-filtering of a baseband signal prior to data decision processing, and to effectively cancel slowly varying DC impairments to the baseband signal.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Digital Transmission Methods That Use Modulated Carrier Waves (AREA)

Claims (8)

  1. Verfahren zum Korrigieren einer Schwankung in einem Gleichstromniveau eines Basisbandsignals zur Verwendung mit einem Kommunikationsdemodulator, in welchem ein moduliertes Kommunikationssignal in ein Basisband untersetzt wird und als Basisbandsignal mit einem Signalverarbeitungsschaltkreis Wechselstrom-gekoppelt wird, wobei das Verfahren die Schritte umfaßt:
    (a) Koppeln des Basisbandsignals mit dem Signalverarbeitungsschaltkreis (40), um eine in dem Basisbandsignal enthaltene Modulation abzuleiten;
    (b) Differenz-Kombinieren (50) des Basisbandsignals und des Ausgangssignal, um ein Breitband-Fehlersignal abzuleiten;
    (c) Integrieren (60) des Fehlersignals, um eine Darstellung eines Korrektursignals der akkumulierten Schwankung des Fehlersignals abzuleiten; und
    (d) Differenz-Kombinieren (70) des Korrektursignals mit dem Basisbandsignals, um die akkumulierte Schwankung in dem Gleichstromniveau aus dem Basisbandsignal zu entfernen.
  2. Verfahren nach Anspruch 1, das weiterhin einen Schritt (e) einbezieht, der umfaßt:
    Tiefpaßfiltern des von dem Signalverarbeitungsschaltkreis (40) abgeleiteten Ausgangssignals, um eine niederfrequente Signalkomponente des Basisbandsignals, das durch das Wechselstromkoppeln des Basisbandsignals mit dem Signalverarbeitungsschaltkreis entfernt wurde, wiederherzustellen, und
    Kombinieren (70) der wiederhergestellten niederfrequenten Signalkomponente des Basisbandsignals mit dem Basisbandsignal, bevor es in dem Signalverarbeitungsschaltkreis (40) verwendet wird.
  3. Verfahren nach einem der vorangegangenen Ansprüche, wobei der Signalverarbeitungsschaltkreis das Ausgangssignal entsprechend eines Vergleichs des Basisbandsignals mit zumindest einem Quantisierungsschwellwert ableiten kann.
  4. Signalverarbeitungsanordnung zum Korrigieren einer Schwankung in einem Gleichstromniveau eines Basisbandsignals zur Verwendung mit einem Kommunikationsdemodulator, in dem ein Kommunikationssignal in ein Basisband untersetzt wird und als Basisbandsignal mit einem Signalverarbeitungsschaltkreis (40) Wechselstrom-gekoppelt wird, wobei der Signalverarbeitungsschaltkreis eine in dem Basisbandsignal enthaltene Ausgangssignaldarstellung einer Modulation ableitet und die Signalverarbeitungsanordnung umfaßt:
    einen Nach-Verarbeitungs-Kombinierer (50), der das Basisbandsignal und das Ausgangssignal differenz-kombinieren kann, um ein Breitband-Fehlersignal abzuleiten;
    einen ersten Bruchteils-Bandbreitenrückkopplungspfad (56), der einen Integrator (60) enthält, welcher das Fehlersignal integrieren kann, um eine Korrektursignaldarstellung der akkumulierten Schwankung des Fehlersignals abzuleiten; und
    ein Vor-Verarbeitungs-Kombinierer (70), der das Korrektursignal mit dem Basisbandsignal differenz-kombinieren kann, um die akkumulierte Schwankung des Fehlersignals des Basisbandsignals vor dessen Verwendung in dem Signalverarbeitungsschaltkreis (40) zu entfernen.
  5. Signalverarbeitungsanordnung nach Anspruch 4, die weiterhin einen zweiten Bruchteils-Bandbreitenrückkopplungspfad (58) umfaßt, der ein Tiefpaßfilter (80) enthält, an das das von dem Signalverarbeitungsschaltkreis (40) abgeleitete Ausgangssignal angelegt wird, um eine niederfrequente Signalkomponente des Basisbandsignals wiederherzustellen, welche durch die Wechselstromkopplung des Basisbandsignals mit dem Signalverarbeitungsschaltkreis (40) entfernt wurde, und wobei der Vor-Verarbeitungs-Kombinierer (70) die wiederhergestellte niederfrequente Signalkomponente des Basisbandsignals mit dem Basisbandsignal vor dessen Anlegen an den Signalverarbeitungsschaltkreis (40) kombinieren kann.
  6. Signalverarbeitungsanordnung nach Anspruch 4 zur Verwendung mit einem Kommunikationsdemodulator, in dem ein moduliertes Kommunikationssignal in das Basisband untersetzt wird und hochpaßgefiltert wird, bevor es als dieses Basisbandsignal mit dem Signalverarbeitungsschaltkreis gekoppelt wird, um die in dem Basisbandsignal enthaltene Modulation abzuleiten, wobei die Signalverarbeitungsanordnung zusätzlich umfaßt:
    ein Tiefpaßfilter (80), das das von dem Signalverarbeitungsschaltkreis (40) abgeleitete Ausgangssignal tiefpaßfilteren kann, um die niederfrequente Signalkomponente des Basisbandsignals wiederherzustellen, welche durch Hochpaßfiltem des Basisbandsignals entfernt wurde.
  7. Signalverarbeitungsanordnung nach Anspruch 6, wobei das Tiefpaßfilter eine obere Grenzfrequenz hat, die im wesentlichen einer unteren Grenzfrequenz eines Hochpaßfilters des Kommunikationsmodulators entspricht.
  8. Signalverarbeitungsanordnung nach einem der Ansprüche 4 bis 7, wobei der Signalverarbeitungsschaltkreis (40) das Ausgangssignal entsprechend des Vergleichs des Basisbandsignals mit zumindest einem Quantisierungsschwellwert ableiten kann.
EP97910903A 1996-10-08 1997-10-08 Verfahren und einrichtung zum ausgleich von gleichspannungsverschiebungen und wiederherstellen der niederfrequenten komponenten eines basisbandsignals Expired - Lifetime EP0865699B1 (de)

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Application Number Priority Date Filing Date Title
US72735096A 1996-10-08 1996-10-08
US727350 1996-10-08
PCT/US1997/018392 WO1998016039A1 (en) 1996-10-08 1997-10-08 Method and apparatus for cancellation of dc offset and reconstruction of low frequency components of baseband signal

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EP0865699B1 true EP0865699B1 (de) 2003-04-02

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US6415003B1 (en) * 1998-09-11 2002-07-02 National Semiconductor Corporation Digital baseline wander correction circuit
GB2346777B (en) * 1999-02-12 2004-04-07 Nokia Mobile Phones Ltd DC offset correction in a direct conversion receiver
GB2355900B (en) 1999-10-29 2004-03-17 Ericsson Telefon Ab L M Radio receiver
US6657476B1 (en) 2002-07-09 2003-12-02 Honeywell International Inc. AC-coupled sensor signal conditioning circuit

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NL7902093A (nl) * 1979-03-16 1980-09-18 Koninkl Philips Electronics Nv Zelfinstellend filter met een vertragingsschakeling.
DE3132972A1 (de) * 1981-08-20 1983-03-24 Siemens AG, 1000 Berlin und 8000 München Regenerator fuer digitale signale mit quantisierter rueckkopplung

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EP0865699A1 (de) 1998-09-23
DE69720414D1 (de) 2003-05-08
DE69720414T2 (de) 2004-01-08

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